Recently a long-term bone marrow culture system has been developed. This system provides a technique for the maintenance of an active, self-renewal population of pluripotent hematopoietic cells (stem cells) and is dependent upon the presence in the culture vessels of a layer of marrow-derived adherent cells (stromal cells). Among stromal cells, only the reticulum cell or adventitial reticular cell is capable in the support of maintenance of stem cells. However, growth of reticulum cells is extremely difficult. A type of fibroblastoid cell was recently proposed to be the reticulum cell, but the cell did not possess hematopoietic activity in its highly purified form. During my cinemicrographic studies on long-term growth of bone marrow cells, a type of net-like cells appeared in the cultures. The cells were maintained in good condition for a period of as long as 44 hours. The cells were very large. The cytoplasm was very thin, but enforced by thick cytoplasmic strands arranged in a confirguration similar to a fishing net. Appearance and disappearance of holes in the cytoplasm were observed. The hole started as a small clear area in the cytoplasm, expanding steadily into a large, round or oval, vacuol; then the size of the hole gradually reduced and finally disappeared without any damage of the cytoplasm. Repeated hole formation and multi-hole formation were observed. The cells remained stationary most of the time, and did not make abrupt contractions as did the fibroblast in cultures. There were one or two clear nuclei, each containing a prominent nucleolus. The nucleus was stable and did not show oscillation or rotation as did the fibroblasts in cultures. All these features suggest that this new hole-forming, net-like cell might be the true reticulum cell of bone marrow. Moreover, my early work on the growth of granulocytes in culters has been recognized by authorities as the first study of stromal cells in marrow cultures.